Adherents.com

"...growing collection of over 41,000 adherent statistics and religious geography citations -- references to published membership/adherent statistics and congregation statistics for over 4,200 religions, churches, denominations, religious bodies, faith groups, tribes, cultures, movements, ultimate concerns, etc.

Basically, researchers can use this site to answer such questions as "How many Methodists live in Indiana?", "What are the major religions of India?", or "What percentage of the world is Hindu?" We present data from both primary research sources such as government census reports, statistical sampling surveys and organizational reporting, as well as citations from secondary literature which mention adherent statistics."